Decoupling of U-Pb and Trace Element Behavior of Rutile During High-temperature Metamorphism

Abstract:

The ability to couple rutile crystallization temperatures using the Zr-in-rutile thermometer with U-Pb age data makes this mineral an outstanding ‘petrochronometer’. However, the relatively low closure temperature of Pb diffusion in rutile (~450°C) compared with the high closure temperature for Zr diffusion suggests we should see a decoupling of these systems. This talk will present two examples from high- to ultrahigh-temperature (HT/UHT) metamorphic terranes in which the timing of rutile crystallization recorded by Zr thermometry is decoupled from cooling ages recorded by U-Pb geochronology.

In example 1, from the Gruf Complex of the European Central Alps, rutile grains in different metapelitic assemblages preserve Zr-in-rutile temperatures indicating prograde metamorphic growth (500-880°C from inclusions in garnet), growth during garnet-breakdown to Spr + Opx ± Crd at UHT conditions (>900°C), and subsequent growth during a lower granulite facies overprint during exhumation (~750°C). This cycle of growth is interpreted to have occurred during a single P-T loop between >35 Ma and ~30 Ma. However, the U-Pb system in rutile only preserves evidence for cooling through Pb-closure at ~21-24 Ma, indicating cooling rates above 30°C/m.y. during assembly of the Gruf Complex geological units and partial exhumation.

In example 2, from the Rayner Complex of the East Antarctic Shield, rutile grains in metasedimentary rocks occur in textural equilibrium with HT-UHT metamorphic assemblages formed at ~930-900 Ma. Zr-in-rutile temperatures typically range from 720-780°C (up to ~850°C) across a range of mineral assemblages, indicating growth prior to peak conditions. However, U-Pb ages lack evidence for the older event, recording a thermal overprint that cooled through closure at ~520-510 Ma, despite large grain diameters up to 500 µm. This younger age is inferred to reflect moderate reheating of the crust in the absence of significant dynamic recrystallization, and can be linked to more intense Pan-African tectonism elsewhere in the region.

These two examples illustrate the decoupled nature of rutile thermometry and geochronology. Zr-in-rutile thermometry can reveal complex thermal histories of polycyclic terranes, whereas U-Pb geochronology typically records only the timing of cooling in HT/UHT rocks.